The arrival of June has already seen the temperatures rise and the humidity start. With air-conditioning in short supply here, the best way to escape the heat is to pack the sunscreen and towel and head to the beach. In Barcelona there are three main places to layout and bake before hitting a chiringuito. Starting with Barceloneta near the port, you can walk north to la playa Icaria by the Hotel Arts and Manfre buildings. Just beyond it is la playa Marbella, where you'll find the only nudist beach in the city. Not natural beaches, they were created as part of the Olympic games to offer those staying in the city easy access to the water, and on summer days they are often packed to capacity, so get there early. Also remember to be careful with your belongings.
If you're looking for something more relaxing, thirty minutes north are las playas de Maresme. You can get there by train from Plaça Catalunya and the fares aren't that expensive. In fact up to Montgat Nord, you can use a standard metro pass. They'll still be crowded on a hot summer's day, but less so than the ones in the city, and you can be a little less paranoid about your belongings. Of these I particularly enjoy Cabrera de Mar and Caldes d'Estrac for a quick day trip, while Santa Susanna is a quaint little beach town that's perfect for a weekend out of the city.
bus from the Arc de Triomf metro station, and it'll run you about an hour or two, depending on where you're heading. The first of these beaches are Lloret and Tossa del Mar, which are particularly popular with the British so be warned, while further north are Palafruguell, L'Estartit and Roses. As you can see, the water is crystal blue and the settings awe-inspiring, but there is little in the way of space to sunbath due to the rocky nature of the coast. Still, it's well-worth a visit and a great place to snorkel or scuba-dive, especially las Islas Medas.
South of Barcelona also offers some fantastic places to layout and enjoy the sun. Different than the rocky coast of la Costa Brava, they tend to offer lots of space and sand. The most famous of these is Sitges, which is just under two hours away. A typical Spanish beach town with white houses and tiny streets, its wide sandy beachs and the shallow water are perfect for those of you with small children. Further south are las Playas of Tarragona, with the most popular being Salou, which personally reminds me of Benidorm. To get to either of these, you'll again catch the train, while all along the coast are more natural and wild beaches such as La Playa Waikiki or Altafulla that'll require renting a car.
To stay in any of the aforementioned places, you can either stay in a hotel, rent a flat or semi-rough it at a camping ground. Also as requested, here's a list with all the festivals taking place in Barcelona including the neighborhood fiestas